The present invention relates to a poly-phase filter and a filter apparatus which compensates for timing and phase error in a digital signal and a method for implementing such compensation. More particularly, the invention relates to a poly-phase filter which compensates for a phase error, a timing error, and a group delay characteristic occurring in a time period which is shorter than the sampling period of sampled data. Additionally, the invention relates to a method for providing such compensation when using a poly-phase filter.
Increased recording density of storage media requires a relatively fast processing speed. Therefore, a digital signal processing method is used instead of an existing analog signal processing method. In a recorder or transmitter, image and audio signals are sampled and converted into digital signals and the digital signals are coded and transmitted or stored in the recording medium.
In a reproducer or a receiver, the transmitted signal undergoes digital signal processing and is reproduced to convey information of the original signal. As a result of using digital techniques, problems associated with sampling phase error and a timing error correction occur. For example, when a digital PLL (phase locked loop) circuit is used for signal correction, it is difficult to realize the digital PLL circuit which requires a high degree of accuracy since the digital PLL has a high operation frequency and a feedback loop.
During digital signal processing, a group delay circuit having a particular sampling period can be constituted using a delay device such as a D flip-flop. However, a circuit in which the group delay is shorter than the sampling period can only be constituted using a filter. The most widely known circuit in which the group delay is shorter than the sampling period is a Hilbert filter having a phase shift of 90.degree.. When the sampling period is normalized, the Hilbert filter has a group delay equivalent to 1/2 of the sampling period. The group delay of the filter is the characteristic showing how long that the signal passed through the filter is delayed. Generally, the group delay corresponds to differentiated phase characteristics of the filter. Here, a phase error follows the same concept as the group delay error, the only difference being the value (phase or time) used as a mediation variable. When the phase error is generated in a present input signal, the signal itself falls behind or precedes a reference time for the signal.
Generally, in the digital filter, the phase error is compensated for by designing a compensating filter using an inverse Fourier transform according to frequency and phase characteristics of a group delay. However, this method cannot be applied to a system in which the group delay characteristics vary over time.